Classification of stainless steel and reasons for its corrosion resistance

Stainless steel is typically classified into five types based on its microstructure. The first type is ferritic stainless steel, with a chromium content ranging from 12% to 30%. Its corrosion resistance, toughness, and weldability increase as the chromium content increases, and its resistance to chloride stress corrosion is also superior to that of other types of stainless steel. 

The next type is austenitic stainless steel. The chromium content in this type of steel needs to be higher than 18%, and it should also contain about 8% of nickel and small amounts of molybdenum, titanium, nitrogen, etc. This steel type has excellent comprehensive performance and can resist corrosion from various media. 

Another type is austenitic-ferritic duplex stainless steel, which combines the advantages of austenitic and ferritic stainless steels and also possesses superplasticity. Additionally, there is martensitic stainless steel, which has high strength but poor plasticity and weldability. 

Finally, there is the precipitation hardening type stainless steel. This steel type has excellent forming properties and weldability, and can be used as a high-strength raw material in the nuclear industry, aerospace and aviation industries. 

The reason why stainless steel is resistant to corrosion 

Usually, metals react with oxygen in the air, forming an oxide layer on their surface. For ordinary carbon steel, the formed iron oxide on the surface will continue to oxidize, causing the rusting to expand continuously, eventually resulting in metal holes. 

The surface of carbon steel can be protected against corrosion by electroplating it with paint or a corrosion-resistant metal. However, this protective layer is merely a thin film. If this protective layer is damaged, the underlying steel will start to rust. 

Chromium is the fundamental element that gives stainless steel its corrosion resistance. When the chromium content in stainless steel reaches 12%, the chromium in the steel will react with oxygen in the external corrosive medium, forming a thin but dense oxide film (self-passivating film) on the surface of the stainless steel. This tightly adhered chromium-rich oxide protects the surface and prevents further oxidation. 

This oxide layer is extremely thin, allowing the natural luster of the steel surface to be seen through it, giving stainless steel its distinctive appearance. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself, re-forming this oxide "passivation film" and continuing to provide protection. This prevents the base of the stainless steel from being further corroded. 

Apart from chromium, the commonly used alloying elements include nickel, molybdenum, titanium, niobium, and copper, which can meet the requirements for the microstructure and properties of stainless steel for various applications. Therefore, the chemical composition of stainless steel has a common feature: the chromium content is all above 10.5%.